By R. A. DAWBARN, M.I.C. E., M.I.E.E. Just 600 years ago the British Parliament successfully petitioned the King to prohibit the use of coal in London, from which time its consumption gradually increased, but it is only within the lives of living men that the great demand for it has arisen for the generation of mechanical power, with which we are for the moment more directly concerned. It is almost startling to recall the fact that only 70 years have elapsed since mail coaching was at its height—a zenith represented by 54 coaches throughout England, together unable to carry as many passengers as a single railway train to-day. But it is perhaps still more remarkable that 20 years from the height of its prosperity sufficed to entirely supersede, the mail coach * and to establish the age of mechanical power. Closely following the spread of railways came that rapid development of trade, demanding the use of power for almost every manufacturing industry, and with this demand a corresponding increase in the consumption of coal, until, at the present time, its output in England is fully 7 tons per annum per head of population. But the consumption of coal in the Transvaal-chiefly for the generation of power-already exceeds 10 tons per head of white population, and anything which affects economy of fuel cannot fail to be of importance to South Africa. Although the demands for power have been increasing with marvellous rapidity for half a century, singularly little advance has been made since the days of Watt in reducing the consumption of coal per unit of mechanical energy obtained from it, in spite of the realization of the fact that manufacturing countries must inevitably lose some all-important industries so soon as the cost of coal is seriously increased by the necessity for obtaining it from greater depths. It is both difficult and expensive to provide means for accurately recording the average power consumed in factories in which many power-using tools are intermittently employed, except where electric motors are in use when accurate records of the energy absorbedhowever intermittent the load-can be obtained automatically, by the use of meters. It is therefore only since the establishment of electric distribution of energy that accurate costs of generating and distributing power have been systematically recorded. On setting to work the earlier electric power stations, it was a surprise to most engineers to find how large the consumption of fuel. was per unit delivered to the consumer. Consumptions of coal as high as 15 lbs. and more per unit sold, with non-condensing engines, were not uncommon, whilst 12 lbs. per unit sold was frequently experienced with condensing engines. NOTE. The first mail coach ran in 1784. The height of coaching was reached in 1838. The last mail coach from London ceased running in 1856. Even to-day there are few electric power stations in England consuming less than 6 lbs. of coal per unit sold. Of the 26 electric supply undertakings in the London Metropolitan area, supplying over 150,000,000 units a year, the average cost of coal in 1905 exceeded 0.5d. per unit sold. Assuming the average cost of coal to be 15/- per ton, this corresponds to an average consumption of 61 lbs. per unit sold. The average thermal value of the coal probably exceeds 13,500 B.Th. U. per lb., on which basis 84,375 B.Th.U. are thus expended per electrical unit sold in London, whereas an electrical unit corresponds to 3,438 B.Th. U., consequently the overall thermal efficiency, that is to say, the proportion of the latent heat energy contained in the coal, which is recovered in the form of electric energy at consumers' premises throughout London, practically 4 per cent. is By employing a small number of correspondingly larger engines of the most economical type, this efficiency ought, in the light of our present knowledge, to be greatly improved, but it is doubtful whether, with the most economical steam plant, and with the highest load factor obtainable under practical conditions, it is possible at the present time to generate and distribute electric energy 24 hours per day over a large area-involving extra high pressure mains and the consequent transforming losses-with a higher overall thermal efficiency than 8 per cent. The following figures show approximately the distribution of losses, giving this result : In view of the last-mentioned loss, it may at first sight be difficult to realize that a power-user, developing his own power direct by modern engines, could under any circumstances be supplied from a distant steam-power station at a lower price than his own cost, with profit to the supplier, for the following reasons: (a) The Power Distributor must (generally speaking) generate his own power in similar manner, and can only obtain comparatively small advantage in the higher efficiency of larger generating units. (b) The Power Distributor has to incur the additional losses of converting his mechanical power as given off by the engines, into electrical power. He has also to incur losses in transmission, and must cover the consumers' losses in reconversion of the electrical energy into mechanical energy. (c) The Power Distributor must incur heavy capital outlay in dynamos and distributing mains, which the poweruser, employing his own direct power plant, avoids. The author's object is to show, firstly, why it is possible under certain conditions to supply fairly large power-users by electrical distribution from a distance with advantage to both supplier and supplied, and, secondly, to point in general terms to the limitations of electric Power Distribution. The advantage of the Power Distributor over the ordinary power-user employing his own plant, may be briefly described under the following headings: (1) Large output. (2) Low Diversity factor. (3) Large Station load factor. 1. LARGE OUTPUT. It will be readily understood that as there are many charges which do not increase pro rata with the output-such, for instance, as management, attendance, and even stores and repairs to a lesser extent the larger the service from one generating station (within limits) the lower will be the cost per unit generated. Larger generating units are, moreover, more economical in fuel consumption than smaller ones, and require less attendance in proportion to the output. 2. DIVERSITY FACTOR. The diversity factor, however, plays a still more important part in establishing the advantage of electrical distribution of power. This may be best explained by a reference to a specific case :— The Natal Government Railways have an electric generating station supplying power to their railway workshops at Durban. There are no less than 406 motors in use, arranged to drive a corresponding number of tools of various kinds. No motor is any larger than is necessary to drive the particular machine to which it is attached. The total power required to serve the whole of these motors at once on full load would be 2000 K.W., but it is found in practice that the maximum power required to supply their aggregate requirements never exceeds 500 K.W. This is due to the fact that without design a number of motors are always at rest for one cause or another, whilst other motors are for the time being required to give less than the maximum power which the tools they drive may at times demand. It follows that in such a case the capacity of the generating station can be reduced to about 25 per cent. of that which would be required to work every motor at full load at one time. This percentage is called the diversity factor. Although in this case the motors are all in one factory, the principle would be the same if every motor represented a separate factory. It is clear that the aggregate capital expended by the many proprietors in order to provide themselves with steam engines and boilers, or other sources of power, to the total capacity of the motors, would greatly exceed the capital to be spent on one generating station of only 25 per cent. of the total capacity comprising a small number of much larger and more economical generating units. The cost of working a large number of scattered small engines and boilers as compared with a few large ones in one building, would also be much greater. 3. STATION LOAD FACTOR. In the case just cited, the working hours are the same for the whole factory; but when supplying a large number of consumers having very different businesses and different working hours, the duration of the average load will be extended. A generating plant, capable of developing 1000 H.P., would, of course, be capable of developing 24,000 H.P. hours per day, but if the aggregate load per day corresponds to only 8,000 H.P. hours, or one-third of the possible maximum-although the maximum power taken at some time during the day was 1000 H.P.-the load factor is said to be 33 per cent. In other words, the station load factor is a fraction of which the numerator is represented by the units generated in a given time, and the denominator by the product of the observed maximum load in Kilowatts, and the total number of hours in the given period-usually one year to cover the variations of the seasons. The longer the machinery can be fully employed, the less, of course, will be the cost per unit generated, owing to the capital charges, management, and other costs, being reduced per unit, pro rata with the output. 4. PLANT LOAD FACTOR. As a result of the diversity factor, it follows that the average load on a generating station, supplying a number of power-users, must be much nearer the maximum load than in the case of each individual motor, and it is well-known that a steam engine lightly loaded consumes considerably more steam per unit generated than at full load. It is the practice in all generating stations to keep only so many of the generating units running as will cope with the total load for the time being. By this means it is frequently found possible, where the station load factor is, say, 25 per cent., to keep the plant load factor as high as 50 per cent., that is to say, the machinery in use is on an average half-loaded. By increasing the number of generating units indefinitely, it would, of course, be theoretically possible to approximate closely to a 100 per cent. plant factor, but this would involve smaller and less economical units, greater capital outlay in plant and buildings, and greater attendance and repair charges. In practice, about five running sets, and a sixth as a stand-by, is found to give economical results, though generating stations are usually started with less generators to allow for increased demands without involving an undue number of generators later. In estimating the cost of generating and distributing electric energy for any given case, one may calculate capital outlay, fuel and labour charges very closely, but the load factor and diversity factor, which have such an important influence on the working cost per unit and on the capital outlay respectively, can only be more or less closely approximated, according to the experience and judgment of the estimating engineer. EFFECT OF LOAD FACTOR. In England the load factor for lighting alone during 1905 was in one case as low as 5.75 per cent. There are 12 undertakings having load factors below 8 per cent., and 37 below 10 per cent. The average is about 15 per cent. Stations which generate electric energy for both tramways and lighting have load factors usually between 20 per cent. and 25 per cent., but Salford has a load factor of 28.55 per cent., and South London 29.32 per cent. None reach 30 per cent. A station supplying tramways only may have a load factor as high as 52 per cent. to 53 per cent., but the author is not aware of any public electric supply stations with a load factor as high as 60 per cent. The effect of the load factor on the total cost of supply, including interest on capital, is shown by the following Table A. : TABLE A. Shewing the effect of "load factor" on the cost of Electric energy supplied from a Public Supply Station. |